6772
Microwave Spectrum, Structure, Dipole Moment, and Quadrupole Coupling Constants of Aminodifluorophosphine' A. H. Brittain, J. E. Smith, P. L. Lee, K. Cohn, and R. H. Schwendeman* Contribution from the Department of Chemistry, Michigan State University, East Lansing, Michigan 48823. Received April 27, 1971 Abstract: The microwave spectrum of PFzNHQis consistent with a molecule containing a planar nitrogen configuration with the hydrogen atoms in the symmetry pl5ne. The structural parameters are : d(PF) = 1,587 0.004 LFPF = 94.6 i 0.2"; d(PN) = 1.650 i O.OO$ A, L F P N = 100.6 f 0.2"; d(NH,i,) = 1.002 i0.005 A, L H N H = 117.2 =t0.4"; d(NH,,,,,) = 0.981 + 0.005 A LPNH,i, = 123.1 i 0.2", LPNHt,,,, = 119.7 0.4". Quadrupole coupling constants for the I 4 N nucleus in PF2I4NHzhave the values xna= 1.7 f 0.1, X b b = - 3.5 f 0.3, a n d x.. = 1.8 f 0.3 MHz. The dipole moment components for PFz14NH2were determined to be p a = 2.570 i 0.007, pC = 0.18 i 0.01 D and yield a total dipole moment of 2.58 i 0.01 D which is inclined a t a n angle of 25.1 O to the P N bond. T h e proton nmr spectrum of liquid PF215NH2 contains a single pair of transitions. The splitting, attributed t o l5N-H coupling, is 83.2 Hz, which is close t o the value expected for a planar -NHz group. N o further splitting is observed down to -70", suggesting that o n the nmr time scale the molecule is undergoing a rapid intramolecular conversion, probably an internal rotation about the P-N bond.
+
A,
*
T n recent years there has been considerable discussion about the nature Of the P-N bond' Some Of the physical and data for compounds containing PN b o n d s have been rationalized b y Postulating the existence of a (p-d)n bond arising f r o m delocalization of the l o n e pair of electrons on the nitrogen atom into empty d orbitals of the p h o s p h o r u s The prepared compound aminodifluorophosphine, P F z N H ~appeared ,~ to be an excellent ex-
Experimental Section The aminodifluorophosphine used in this study was prepared by the reaction of ammonia with pFzC1.6 The ammonia (Matheson) was dried over sodium before use. The PF2Clwas prepared by the reaction of (CH3)2NPF2with HCI.' Samples enriched in PFzl6"H2 and PFzNDz were prepared by reaction of PFZCl with I6NH3and ND3, respectively. A sample containing a mixture of PFzNHD species was prepared in situ by exchange of PFzNHzwith D?O in the waveguide sample cell. The various samples were characterized by infrared, mass, and nmr spectra.
Table I. Hypothetical Unsplit Frequencies" for Isotopic Species of PF2NH2
Transition ooo-~ol
110-211 111-212 101-220 101-202
PFz'~NH~ 11648.76 0.04b 25753.19 -0.09 20841.47 -0.09 0.05 32941.17 21421.15 -0.05
202-221
42a-'b 413-43 a 533-532 52s-542
12703.83 -0.01 14583.47 -0.98 11444.93 0.02 15510.66 -2.51
PF2"NHz
PF2NHD(cis)
11338.70 0.04 25004.55 -0.07 20349.96 -0.07
11160.72 0.06 24495.61 -0.12 20146.3' -0.6
10932.42 24037.23 19692.48
21048.36 -0.11 9975.72 0.10 11838.52 0.10
20827.56 -0.04 9495.95 0.02 11010.20 -0.29
20159.48 -0.13 10526.21 0.13 10673.23 0.29
10106.93
a
In megahertz.
0.00 0.01 0,02
PF2NDz 10514.55 0.06 22962.71 -0.05 19095.19 -0.03 19943.48 -0.03 9177.80 -0.06
0.18 23206.32 -0.55
506-514
744-743 8.54-853
PFzNHD(trans)
9541.35 15404.52
1.09
9065.97
Hypothetical unsplit frequency minus calculated frequency.
ample for investigation o f possible effects of ( p - d ) r Consequently, we have e x a m i n e d the microwave spectrum of the parent compound and f o u r isotopically labeled species. We also e x a m i n e d the proton nmr spectrum of a s a m p l e of liquid PF2l5NH2.
bonding.
(1) This work was supported in part by grants from the National Science Foundation. (2) (a) K. A. R. Mitchell, Chem. Rev., 69,157 (1969); (b) A. H . Cowley, M. J. s. Dewar, W. R. Jackson, and W. B. Jennings, J . Amer. Chem. Soc., 92, 5206 (1970). (3) (a) A. B. Burg and B. J. Slota, ibid., 80, 1107 (1958); (b) G. Ewart, D. S. Payne, A. L. Porte, and A. P. Lane, J . Chem. Soc., 3984 (1962). (4) (a) R. R. Holmes and R. P. Carter, Inorg. Chem., 2, 1146 (1963); (b) W. A. Hart and H. H. Sisler, ibid., 3, 617 (1964). ( 5 ) (a) R. Schmutzler, ibid., 3, 415 (1964); (b) A. H. Cowley and R. P. Pinnell, J. Amer. Chem. Soc., 87, 4654 (1965); (c) A. H. Cowley and M. H. Hnoosh, ibid., 88,2545 (1966); (d) K. Cohn and R. W . Parry, Inorg. Chem., 7, 46 (1968); (e) J. F. Nixon and M. D. Sexton, J . Chem. Soc. A., 1089 (1969); (f) R. M. Kren and H. H. Sisler, Inorg. Chem., 9, 836 (1970). (6) J. E. Smith and K. Cohn, J . Amer. Chem. Soc., 92, 6185 (1970).
Journal of the American Chemical Society
I 93.25
0.28
1.39
Interference from strong line at lower frequency.
The microwave spectra of the samples were quite rich, with a predominance of a-type Q-branch transitions from which initial assignments were made. The assignments were completed by the measurement of the frequencies of several a-type R-branch transitions. No b-type or C-type transitions were identified. The ground-state transitions were accompanied by vibrational satellites, and transitions in several excited vibrational states of PF2I4NHz and PF2I8NNH2were assigned. Quadrupole hyperfine structure was observed on most of the transitions (except for PF2l6NH2), but complete resolution of the hyperfine components was rarely possible because of rather large line widths. The hypothetical unsplit frequencies of the observed transitions for the ground states of the various isotopic species are given in Table I, with the exception of some intermediate J transitions for PFZI'NHZ which are given in Table 11. The corresponding frequencies for two vibrationally excited states of PF2I4NHz and four vibrationally excited states of PF215NH?are shown in Tables 111 (7) J. G. Morse, K. Cohn, R . W. Rudolph, and R. W. Parry, Inorg. Syn., 10, 147 (1967).
/ December 15,1971
6773
Spectral Analysis
Table 11. Hypothetical Unsplit Frequencies" of Intermediate J Transitions of PF214NH2 Transition 533-532 634-633 643-642 753-752 854-853
964-963
Transition
Aub
v
11444.93 0.02 18353.18 -0.83 9798,84 0.46 7903.54 1.18 15404.52 1.09 3.27 13195.68
413-432 523-542 615-634 633-652 734-753 743-762
Y
AlJ
14583.47 15510.66 24940.83 17230.21 19995.52 20016.76
-0.98 -2.58 - 2.13 - 5.25 -6.07 -9.39
The rotational constants, moments of inertia, principal second moments, and quadrupole coupling constants obtained by analysis of the spectra of the groundstate species are given in Table V. The value of the out-of-plane second moment, P b b = Llm,bi2, is seen to be 'almost constant for all the isotopic species, suggesting that the molecule has a plane of symmetry containing the phosphorus, nitrogen, and both hydrogen atoms. The small variations in P b b are attributed to vibrational changes from one isotopic species to another, and their magnitude is consistent with a planar P-NH2 group. In formamide,* nonplanarity of the molecule was suggested upon comparison of the out-of-plane second moments (Pccin this case) of the parent, NHD(trans), NHD(cis), and ND, species. The values of P,, for the trans-D, cis-D, and ND, species were larger t h m that of the parent by 0.0058, 0.0120, and 0.0147 uA2, respectively. The corresponding changes ino P b b for PF2NH2are 0.0025, -0.0081, and -0.0006 uA2. The fact that the changes are small and in two cases negative for PF2NH2 is strong evidence for a planar nitrogen configuration.
In megahertz. Hypothetical unsplit frequency minus calculated frequency (rotatioid constants in Table V). Table 111. Hypothetical Unsplit Frequencies" of Vibrationally Excited States of PF2I4NH2 Transition O~a-loi 110-211 111-212 101-202 533-532 643-642 753-752
v12
11678.55 25852.15 20862.00 21396.98 11857.34 10387.32 8647.55
=
1
v12
11692.30 25901.31 20868.10 21380.98 12063.80 10680.43 9021.45
O.Olb
-0.01 -0.01 0.01 0.00 0.67 1.55
= 2
0.03b 0.17 0.06 -0.25 -0.06 0.45 1.66
a In megahertz. Hypothetical unsplit frequency minus calculated frequency (rotational constants in Table VI).
Table IV. Hvoothetical Unsolit Freauenciesa of Vibrationallv Excited States of PFPNH2 Transition
v12 =
1
Y12
=
2
P11 =
O.Olb 11365.83 25095.54 -0.02
11378.56 0.00 25141.99 0.00
21030.56 -0.03
21018.08 0.00
11346.52 0.24 25040.04 0.19 20345.41 0.11 21027.31 -0.12
9857.75 -0.01
12099.84
1
9926.89 0.04 32828.39 -0.04 34038 71 -0.16 19042.70 0.39 19262.12 0.38
0.02 18466.27 -0.03
10510.85 -0.02 a
In megahertz.
vg
=
1
25011.03 0.16 20279.45 -0.13 20952.15 0.04 32561.64 0.51 38205.38 33967.51 19131.09 19335.10 12094.14 18585.22 10471.54
-0.24 -0.33 0.57 -0.14 -0.18 -0.24 -0.27
Hypothetical unsplit frequency minus calculated frequency (rotational constants in Table VI).
Table V. Rotational Constants," Moments of Inertia,bPrincipal Second Moments,band Quadrupole Coupling Constants" of YF~NHz PF2 4NHz 7766.22 (0.05y 7052.29 (0.05) 4596.43 (0.05) 65.0736 71.6613 109.9498 58.2687 51.6810 13.3926 O.oo00 1.65(0.13) -3.45 (0.25) 1.80 (0.25)
PF216NHz
PFzNHDrcis)
PF2NHD(trans)
PFzNDz
7756.02 (0.05) 6832.98 (0.05) 4505.69 (0.05) 65.1492 73.961 3 112.1640 60.4830 51.6809 13.4783 -0.ooO1
7580.88 (0.08) 6667.53 (0.05) 4493.12 (0.05) 66.6645 75.7966 112.4777 60.8049 51,6729 14.9917 -0.0081 1.656 -3.456 1 ,808
7761.48 (0.15) 6552.40 (0.05) 4380.02 (0.05) 65.1134 77.1284 115.3821 63.6986 51.6835 13.4298 0,0025 1.17(0.25) -3.38(0.39) 2.21 (0.39)
7569.63 (0.15) 6224.13 (0.05) 4290.36 (0.05) 66.7636 81.1962 117.7934 66.1130 51.6804 15.0832 -0.0006 1.656 -3.45e 1.80e
_
_
_
_
~~
In megahertz. In unified atomic mass units (dngstroms)2. Values in parentheses are experimental uncertainties. Pbb(isotopic species) - pbb(pF~~~NH2). e Value assumed from PFzI4NH2. and IV, respectively. The transitions were observed and the frequencies measured with a 100-kHz Stark-modulated spectrometer using backward wave OSCillatOrS as microwave sources. The Rband transitions in the excited states of PF215NH2were examined with a Hewlett-Packard R8460 MRR spectrometer. A search for the spectrum of another isomer of P h N H 2was unsuccessful.
APbb
=
The rotational constants of the two vibrationally excited pF214NH2 and the four vibrationally excited PF~I~NH species , which have been assigned are given in Table VI. These constants were derived from analy(8) C. C. Costain and J. M. Dowling, J . Chem. Phys., 32, 158 (1960).
Brittain, Smith, Lee, Cohn, Schwendeman
Aminodifluorophosphine
6774 Table VI. Rotational Constants" of Vibrationally and PFz16NHz Excited Species of PF2l4NH%
PFz"NH2 PFzl'NHz
U ~ Z=
1 2
U ~ Z=
1
U ~ Z=
2 vi1 = 1 ug = 1 vi2
a
A
B
C
7731.28 7716.20 7722.09 7707.43 7738.48 7717.00
7086.81 7104.42 6864.91 6881.72 6846.78 6844.13
4591.73 4587.87 4500.82 4496.85 4499.51 4478.49
In megahertz.
ses of the frequencies of spectral lines given in Tables I11 and IV. In Tables 111, IV, and V the various vibrational states have been numbered according to the following considerations. If a plane of symmetry is assumed for the vibrational potential function, the 12 vibrational motions divide into eight motions of symmetry A' (in plane) and four of symmetry A" (out of plane). The lowest frequency A' motion has been observed9 in the infrared spectrum at 354 cm-' and is called v8. The lowest frequency A" motion has a measured vibrational frequency of 170 cm-I and is called v12. The next lowest A" motion, seen at 235 cm-l, is referred to as vll. The fourth set of vibrational satellites assigned in the microwave spectrum is attributed to 2v12. Rough measurements of relative intensities give values which are in accord with these assignments. Examination of the rotational constants for the v12 and 2v12 species in Table VI reveals that the rotational constants do not vary in a harmonic manner. We believe that this is due to a vibration-rotation interaction between 2v12 and V S ; the symmetries of the two states are the same and the energy difference is not large (-15 cm-l). The absence of unusual effects in the vibrational satellite pattern appears to confirm a plane of symmetry in PF2NH2. The best evidence for nonplanarity in formamide is probably the identification of a severely anharmonic vibration. No such phenomenon occurs for PF2NH2 unless the anharmonicity is very small. Nevertheless, since some question remains, and since PF2NH2is an unusual molecule, the vibrational assignment and the question of vibration-rotation interaction is currently under investigation in these laborat~ries.~"
Molecular Structure The coordinates of the atoms in the principal inertial axis system of PF,NH2 were determined from the moments of inertia of the five isotropically different species in four ways. (1) The coordinates of the nitrogen and hydrogen atoms were determined by means of the Kraitchman equationslO assuming a plane of symmetry, the 6 coordinates of the fluorine atoms were determined by means of the relation P b b = 2mFbF,2 where m F is the mass of a fluorine atom and Pbbis a principal second moment of the parent species, and the remaining co(9) P. L. Lee, I
